Electrical bushing with stress equalizer mounting arrangement



Oct. 27, 1964 D. L. JOHNSTON 3,154,630

ELECTRICAL BUSHING WITH STRESS EQUALIZER MOUNTING ARRANGEMENT Filed April 3, 1963 United States Patent 3,154,630 ELECTRICAL BUSHING WITH STRESS EQUAL- IZER MOUNTING ARRANGEMENT Donald L. Johnston, Pittsfield, Mass, assignor to Gfenlgral Electric Company, a corporation of New Filed Apr. 3, 1963, Ser. No. 270,317 2 Claims. (Cl. 174-31) This invention relates to electrical bushings and more particularly to improvements in high voltage fluid insulated capacitor bushings.

A capacitor bushing has one or more stress equalizing cylindrical conductive surfaces in the radially and axially extending space between a central high voltage conductor and an outer or surrounding ground sleeve or elec trode of conducting material. In conventional capacitor bushings, the stress equalizer is embedded in a solid insulating core of wound paper which may or may not be impregnated with insulating oil, surrounding the central high voltage conductor. In gas or liquid insulated or filled bushings, separate structural insulating supporting means for the stress equalizer must be provided. The natural place to attach such supporting means to the stress equalizer is at the ends of the latter, but I have found that this produces objectionable creepage on the surface of the supporting means adjacent the ends of the equalizer where the electric stress concentration on the equalizer is highest.

In accordance with this invention, the equalizer of a fluid insulated bushing is supported and positioned by solid structural insulating means attached to the equalizer intermediate its ends so as to reduce the surface creepage stress of said supporting means and so as to increase the length of the surface creepage path.

In accordance with another feature of this invention, the equalizer supporting means is located wholly outside the radial spaces between the equalizer and the high volttage central conductor on the one hand and the equalizer and the outer ground sleeve on the other hand so that the effectiveness of the equalizer and the fluid insulation will not be impaired by having insulating material with a different dielectric constant than that of the insulating fluid in those radial spaces where the radial field intensity is highest.

An object of the invention is to provide a new and improvide high voltage electrical bushing.

Another object of the invention is to provide a new and improved fluid insulated high voltage capacitor bushing.

A further object of the invention is to provide a new and improved means for supporting a stress equalizer in a high voltage gas filled capacitor bushing.

The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring now to the single figure of the accompanying drawing, which is a partly broken away and sectionalized elevation view of a preferred embodiment of the invention, there is shown therein by way of example a so-called center clamped high voltage fluid insulated bushing having a central high voltage conductor 1 which may be a copper tube or rod, surrounded near its center by a mounting flange 2 to which is attached a so-called ground sleeve 3 of cylindrical shape and conductive or metallic construction or material. Effectively capping the ends of the ground sleeve 3 are frustoconical insulating shells 4 and 5 of porcelain or the like. The central conductor 1 is attached to a metallic terminal washer 11 closing the lower end of the shell 5 and to the upper end of the shell 4 by means of conventional compression springs (not shown) in an upper dome terminal assembly 6 in such a way that the central conductor is under mechanical ten- 3,154,630 Patented Oct. 27, 1964 sion and consequently the ground sleeve 3 and the shells 4 and 5 are effectively clamped together under compression. Suitable gasket seals 7 are shown interposed between the ends of the shells 4 and 5 and the mounting flange-ground sleeve sub-assembly so as to provide a fluid tight enclosure which is filled with any suitable insulating fluid such as a likuid in the form of mineral insulating oil or gas such as sulfurhexafiuoride, nitrogen, hydrogen or C F but it should of course be understood that the invention is not limited to the use of any particular insulating gas or liquid. It will be understood of course that suitable fluid-tight seals are also formed between the outer ends of the shells 4 and 5 and the metallic terminal assemblies forming the end caps for the bushing.

As will be seen in the drawing, the bushing has an axis of symmetry corresponding to the central conductor 1 or the imaginary center line of the latter so that the ground sleeve 3 and the insulating shells 4 and 5 all have axes corresponding to that of the high voltage conductor 1.

For equalizing the electric stress in the insulating fluid in the bushing, at least one cylindrical conductive equalizer 8 is provided in the space between the central conductor 1 and the ground sleeve 3. The symmetry of the entire structure is preserved by having the axis of the equalizer correspond with the axis of the central conductor. The length of the equalizer between its opposite ends is less than the length of the central conductor but greater than the length of the ground sleeve 3. The actual length of the equalizer 8 will depend on its radial spacing from the central high voltage conductor and the sleeve 3 at ground potential, it being generally concentric with the latter.

As is well known in bushing practice the electrostatic coupling or capactive effects between the surfaces of the elements 1, 3, and 8 are such that the equalizer will assume or have a potential which is generally proportional to its radial spacing from the central conductor 1 and the ground sleeve 3, i.e. if it is radially halfway between elements 1 and 3, it will have a potential to ground substantially half that of the high voltage conductor 1. The equalizer 8 is shown supported by solid structural insulating means 9 and 10 attached to the equalizer 8 intermediate its ends. These insulating supporting means may be made of any suitable material, such as resin impregnated and bonded cellulosic material (paper for example) or molded plastic epoxide material. Supporting means 9 is shown attached at its other end to the central conductor 1, and supporting means It) is shown seated at its other end upon the conducting terminal washer 11.

By means of this construction the surface creepage path along the solid supporting means 9 and 10 is longer than if those supporting means were connected to the ends of the equalizer 8. Furthermore, the electric stress con centration at those points of connection is less than it would be if the means 9 and 14) were connected to the ends of the equalizer where the discontinuity in the structure causes a piling up or increase in electric stress concentration.

Furthermore, it will be seen that the supporting means 9 and It) lie wholly outside the radial and axial spaces between the equalizer 8 and the center conductor 1 on the one hand and the equalizer 8 and the sleeve 3 on the other hand. In this way, the stress equalizing action of the equalizer 8 is not impaired by distorting the electric field on either side of it opposite the central conductor 1 and the ground sleeve 3 through the presence of dielectric material in those spaces having a dielectric constant different from that of the insulating fluid which fills the bushing. In other words, the point or points of attachment of the supporting means to the equalizer is on the outer surface of the equalizer between its ends and the 3 ends of the ground sleeve so as not to displace fluid insulation in the zones of highest axial and radial field intensity.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications can be made without departing from the invention, and therefore it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed as new and which it is desired to secure by Letters Patent of the United States is:

1. In a fluid-filled electrical capacitor bushing for an elongate high voltage conductor including spaced-apart terminal members, a normally grounded external cylindrical metallic sleeve coaxially surrounding said conductor intermediate said terminal members, a pair of generally frustoconical porcelain shells capping opposite ends of said sleeve between said terminal members, said shells coaxially surrounding said elongate conductor and being sealed to said terminal members and to said sleeve thereby to provide an enclosed tubular space surrounding said conductor, an insulating fluid filling said enclosed space,

a cylindrical conductive electric stress equalizer posi-,

tioned in said space in coaxial spaced relation between said conductor and said sleeve, the axial length of said stress equalizer being intermediate the overall length of said bushing and the axial length of said sleeve, and insulating structural supporting means carried by said conductor and fixed to said stress equalizer intermediate its ends, said supporting means being positioned Wholly outside the radial spaces between said stress equalizer and said conductor and between said stress equalizer and said sleeve.

2. A bushing as defined in claim 1 in which said insulating supporting means is attached to the outer surface of said equalizer intermediate its ends and outside the axial confines of said metallic sleeve.

References Cited by the Examiner UNITED STATES PATENTS JOHN P. WILDMAN, Examiner. 

1. IN A FLUID-FILLED ELECTRICAL CAPACITOR BUSHING FOR AN ELONGATE HIGH VOLTAGE CONDUCTOR INCLUDING SPACED-APART TERMINAL MEMBERS, A NORMALLY GROUNDED EXTERNAL CYLINDRICAL METALLIC SLEEVE COAXIALLY SURROUNDING SAID CONDUCTOR INTERMEDIATE SAID TERMINAL MEMBERS, A PAIR OF GENERALLY FRUSTOCONICAL PORCELAIN SHELLS CAPPING OPPOSITE ENDS OF SAID SLEEVE BETWEEN SAID TERMINAL MEMBERS, SAID SHELLS COAXIALLY SURROUNDING SAID ELONGATE CONDUCTOR AND BEING SEALED TO SAID TERMINAL MEMBERS AND TO SAID SLEEVE THEREBY TO PROVIDE AN ENCLOSED TUBULAR SPACE SURROUNDING SAID CONDUCTOR, AN INSULATING FLUID FILLING SAID ENCLOSED SPACE, A CYLINDRICAL CONDUCTIVE ELECTRIC STRESS EQUALIZER POSI- 